Skis incorporating a core of injected synthetic foam are relatively simple to manufacture.
To present good qualities of mechanical resistance and of performance on snow, a ski must be longitudinally flexible in order to follow the form of the terrain, in particular the hollows and bumps, whilst being sufficiently resilient to resume its initial form rapidly. On the other hand, it must be rathermore rigid in lateral bending in order always to conserve the shape of the line of edges which allows the ski to be controlled correctly in a turn.
By providing reinforcing members within the structure of the ski, and by acting on the nature, shape and positioning of these reinforcements, it is possible to modify the characteristics of this ski to adapt them to the use provided by the specifications: use in short turns, in large-radius turns, in downhill skiing, and the like.
The reinforcing members which may be embedded in the skis incorporating an injected core are, in the present state of the art, generally of the following different types:
They may be metallic and made, for example of steel or aluminium alloy. In this case, they are in the form of plates, longitudinal rods, for example, in the form of piano strings, or of nettings. PA1 They may be fibrous and constituted, for example, by glass, carbon, or aramide fibers. In this case, they are in the form of plates, rods, pre-polymerized nettings, fabrics pre-impregnated with resin, or of dry fabrics. PA1 Ef is the Young's modulus of the reinforcement PA1 b is the width of the reinforcement PA1 d is the distance of the reinforcement to the neutral axis of the ski PA1 e is the thickness of the reinforcement. PA1 preparing an assembly composed of a plurality of superposed, pre-assembled elements including, successively from top to bottom: PA1 the upper part of the ski at least composed of the plate constituting the possibly decorated protecting element of the ski, PA1 said reinforcing fabric pre-shaped as a tile and pre-assembled by adhesive force at a distance from this upper part thanks to the interposition of beads or pellets of a pasty, sticky product, such as a hot melt adhesive; PA1 placing in the lower part of the mould the lower elements of the ski: typically, the edges, sole, possible lower reinforcing plate(s), and the like; PA1 curving the upper assembly in the form of an upturned gutter to fit it in this lower part of the mould; PA1 thereafter closing the mould with its cover of upturned U section, injecting the synthetic matter and, finally, proceeding with the conventional operations of cooling, opening of the mould, extraction, trimming and sanding of the ski.
One of the specific problems encountered in this technolology of manufacturing skis with injected core "in situ" is the holding of the elements in position within the structure during the operation of injection.
Very schematically, the conventional process of manufacture consists of depositing against the walls of the mould the components which constitute the envelope of the ski (the base, edges, top . . . ), then injecting the expansible foam inside the cavity thus formed. It cannot really be claimed that a high-performance ski has thus been produced, as there are lacking inside such a structure the mechanisation elements necessary for responding to the above-mentioned characteristics of resistance and performance. To that end, it is necessary, as indicated hereinabove, to house in the cavity a sufficient quantity of judiciously positioned reinforcing elements.
The precision of the positioning of this type of reinforcement with respect to the neutral axis of the ski (axis on which no deformation is exerted during a bending stress on the ski) is very important, as a slight difference of the distance d which separates the reinforcement from this neutral axis has considerable effect on the rigidity of the ski in simple bending.
In fact, if, in a first approximation, the rigidity of the lateral faces with respect to local bending is neglected, the rigidity D to bending of a ski is expressed by the formula: EQU D=1/2.Ef.b.e.d.sup.2
where:
Finally, it is observed that this rigidity D is a function of the square of the distance d which separates the reinforcement from the neutral axis of the ski.
On the same pair of skis, it is important that the two skis have identical bending characteristics. In addition, these bending characteristics must be the same for all the skis of the same type. Finally, this results in the positioning of the internal reinforcements having to be precise in order to obtain the desired result as well as a good reproducibility of this result.
Injection technology being what it is, it is consequently necessary to find devices for maintaining the internal reinforcement(s) in a precise position during injection, despite the thrust due to the expansion of the polyurethane form necessary for homogeneous filling of the cavity.
This is why it has been provided to embed in the core of polyurethane (or other injectable synthetic matter), a flattened, perforated tube which is shaped from a polymerized resin netting of fairly great rigidity. This perforated tube then extends over virtually the whole length of the ski, roughly between the heel and the beginning of the tip. This tube is simply placed in the mould before the cover is closed. Its constituent netting presenting wide meshes, it offers no obstacle to the passage of the polyurethane and does not undergo substantial deformation during the operation of injection.
However, this reinforcing tube does not prove sufficient to give the ski the optimum quality of rigidity in bending and in torsion which are generally desired.
It is an object of the present invention to overcome all these drawbacks.